Coating aluminum articles



Patented Feb. 10, 1925.

UNITED STATES PATENT OFFICE.

FULTON B. FLICK, OF NEW KENSINGTON, PENNSYLVANIA, ASSIGNOR T0 ALUMINUM COMPANY OF AMERICA, OF PITTSBURGH, PENNSYLVANIA, A CORPORATION OF PENNSYLVANIA.-

COATING ALUMINUM ARTICLES.

No Drawing.

T 0 all whom it may concern Be it known that I, FULTON B. FLIcK, a citizen of the United States, and a resident of New Kensington, in the county of vVestmoreland and State of Pennsylvania, have invented a new and useful Improvement in Coating Aluminum Articles, of which the following is a specification.

The invention relates to the coating of aluminum, whether the aluminum be what is known as commercially pure aluminum or an aluminum base alloy, both being contemplated by the term aluminum as used in this specification and claims. The invention has to do both with coated aluminum articles and with a process for coating aluminum articles, the term articles being used to designate both partly finished objects such as billets, slabs, plates, rods, etc, and finished objects, such as completed products fabricated from aluminum.

t is known that aluminum readily acquires a superficial film of oxide, and such oxide films or coatings have been proposed for various purposes, as for example, for protection against corrosion, for electric .insulation, dielectrics, etc. However, for many purposes for which they, might find use the coatings heretofore known are unsatisfactory in that they are deficient in covering power and in adherence to the articles, and consequently in their protective qualities. Because of their lack of adherence they readily scale or break off from the articles, particularly when they are bent. Furthermore, I have found that the oxide coatings that have heretofore been formed on aluminum are incapable of being satisfactorily colored because the coatings are thin and nonabsorbent.

An object of the invention is to provide a commercially practical process for formin on aluminum articles a dense, absorbent and adherent coating of aluminum oxide.

A further object is to provide a commercially practical process of forming on aluminum articles a dense, adherent coating of aluminum oxide combined with a coloring substance. I

' A further object is to provide an aluminum article having a dense absorbent and adherent coating which is resistant to abrasion and which does not crack or peel oif when the article is bent.

Application filed July 10, 1923. Serial No. 650,732.

A further object is to provide an aluminum article with a dense adherent coatgig combined throughout its depth with a In the practice of the invention, as far as 1t relates to the process of coating without reference to coloring, an aluminum article is provided with a coating of aluminum oxide by employing the article as anode in the electrolysis of an aqueous solution con taining ammonia, and preferably also containing ammonium sulphide. The electrolys1s of the electrolyte may be efi'ected in any desired form of electrolytic cell suitable in size and in current capacity to the requirements of the particular article or articles to be coated.

The amount of ammonia or ammonium sulphide in the electrolyte may vary between rather Wide limits. Electrolytes containing as little as 2% and as high as 25% of ammonia or ammonium sulphide have given good results. As a specific example, the electrolyte may contain one part of concentrated aqua ammonia to ten parts of water by volume. Preferably, however, it contains about one part of ammoniacal ammonium sulphide solution to ten parts of water by volume, the ammonium sulphide solution being made by the absorption of about fifty grams of hydrogen sulphide in one liter of concentrated aqua ammonia. polysulphide is also effective as an electrolyte, but it is not necessary to add sulphur to the solution containing ammonium sulphide. However, polysulphide may be formed in the solution on standing.

The voltage required for forming a thick,

uniform, dense, adherent and otherwise satisfactory coating of aluminum oxide on an articl is in excess of about one hundred and fifty volts. WVhen the voltage is too low the coating may be streaked and may incompletely cover thearticle, or, if a satisfactory coating is ultimately effected under low voltage operation, an excessively long time is required for doing so. Satisfactory coatings have been produced with a current density as low as three and as high as twenty-five or more amperes per square inch of the article being coated. The current density customarily used is so high that heating of the electrolyte solution cannot be avoided unless resort is had to artificial Ammonium cooling. However, hot solutions have been found'to be advantageous in the formation of coatings, the temperature preferably being, from 30 to 90 C.

In carrying out the process, an article to be coated may be, and preferably is, first cleaned in a suitable manner, as for example by immersing it in an aqueous solution containing hydrofluoric and nitric acids in proportions of from about two to five per cent byvolume of the usual concentrated acid. The article cleaned in this or any other suit able manner'is then made anode in an electrolytic cell containing an electrolyte such as explained. i Ithas'been found by way of a specific example, that by using as the electrolyte an ammonium sulphide solution of the proportions stated, and by using anode and cathode areas of six square inches each, and a potential difference of two hundredand twenty volts between the electrodes, the initial current destiny is twelve amperes per square inch, and a very satisfactory coating of aluminum oxide is completely formed on the anode article in from one to two minutes. Under these con ditions the current destiny falls off rapidly .as the coating is formed and is only about 2.5. amperes per square inch at the end of two minutes.- The aluminum this process is uniform over the entire surface of the article and has a smooth velvety appearance which makes a leasing finish. It is hard, resistant to abrasion, and so adherent to the. article that it does not scale off or crackwhen the article is bent. Furthermore, it shows remarkable protective action under conditions which usually cause marked corrosion of aluminum. The protective action of the coating was shown by a test in which a sample of aluminum coated according to this electrolytic process and a sample coated according to the process of simple immersion in aqua ammonia were covered with sawdust moistened with a mixture of salt and acetic acid. After being so treated for ten days, the aluminum sample coated according to this process was practically unchanged in appearance, whereas the other sample was deeply pitted.

Being characterized in the manner ex-v plained, the coating is peculiarly well suited for protecting aluminum from corrosion, for electric insulation, as a dlelectrlc material, for rectifiers, condensers, etc. Furthermore, it has been found particularly well adapted for preventing seizing of aluminum bearing surfaces in the manner disclosed in United States patent application Serial No. 634,228, filed April 24, 1923, by Cyril S. Taylor. For all purposes the process has the very advantageous feature that requires only a couple of minutes to form an oxide coating of the characoxide coating formed by ter explained, whereas in the formation of inferioroxldes by the prior art processes extended periods of time may be required.

As already indicated, the invention furthcr contemplates a colored coating for aluminum articles. Incidental to the process that has been explained different colored coatings may be formed depending upon the particular electrolyte solution used, and also upon the constituent elements which form an aluminum base alloy to be coated. The use of an electrolyte containing ammonium sulphide gives a coating which is bluish-gray in color. When the article being coated is an aluminumscopper alloy, the coating usually has a decided greenish tinge. By omitting the sulphide from the electrolyte solution, a light cream-colored coating is produced.

As an additional feature of the invention, it has been found that the coating produced by the process has a further unique characteristic, namely that of being absorbent,

which permits it being permanently dyed to produce a wide variety of colors. 4 There are many dyes, usually acidic in nature,'which combine with aluminum hydroxide to form an aluminum salt of the type known as a lake. I have discovered that such dyes maybe combined with or absorbed on this oxide coating, either while the coating is being formed by adding a dye to the electrolyte, or after it is formed by immersing the coated article in a solution of a dye. For example, by using an ammonium sulphide solution as an electrolyte and adding to it from about 0.1 to 0.3 gram benzopurpurin per liter of the solution, the coating resulting from the process is a pleasing shade of red. The addition of a logwood extract to the electrolyte gives a blue coating, cochineal a pink coating, and other dyes various other colored coatings, it being understood that by the proper selection of suitable dyes almost any desired color of coating may be produced.

WVhile it is generally preferred to add the dye to the electrolyte and thereby simultan eously coat and color the surface of an aluminum article, the color may, as indicated, be effected by immersing the article in a dye solution. This is found'preferable in cases where the presence of a dye in an ammoniacal solution is not desirable. When the coating is eifected by immersion, the combination of a dye with the oxide coating frequently may be facilitated by heating the dye solution to form from about 50 to 80 C.

The coloring of the surface of an aluminum article produced according to this process is as deep as the oxide coating, and in appearance it is smooth and velvety, giving the article a pleasing finish.

Reference has been made to dyes which combine with aluminum hydroxide to form an aluminum salt known as a lake, and it has been explained that such dyes may be used to color the oxide coating of an aluminum article. Although lakes are commonly formed by combination with or absorption of dyes by a wide variety of inorganic substances, the lake-forming dyes contemplated by this invention are only those which can combine with or be absorbed by aluminum oxide such as is formed by the process herein disclosed.

According to the provisions of the patent statutes, I have described the principle and characteristics of my invention together with the preferred manner of practicing it and several variations and modifications thereof. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than specifically described herein.

I claim as my invention:

1. The process of forming a coating of oxide on an aluminum article, which comprises employing the article as anode in the electrolysis of an aqueous solution containing ammonia.

2. The process of forming a coating of oxide on an aluminum article, which comprises employing the article as anode in the electrolysis of an aqueous solution containing ammonia and ammonium sulphide.

3. The process of forming a coating of oxide of aluminum on an aluminum article, which comprises making the article anode in an electrolytic cell containing an aqueous solution including ammonia, and passing an electric current of initial high density through the article.

4. The process of forming on an aluminum article a dense, absorbent and adherent coating of oxide of aluminum, which comprises employing the article as anode in the electrolysis of an aqueous solution containing ammonia and ammounium sulphide.

5. The process of coloring the surface of aluminum, which comprises providing the aluminum with an absorbent and adherent coating of oxide of aluminum and treating the aluminum with a lake-forming dye.

6. The proces of coloring the surface of aluminum, which comprises simultaneously providing the aluminum with an absorbent and adherent coating of oxide of aluminum and treating it with a lake-forming dye.

7. The process of providing an aluminum article with a colored coating, which comprises employing the article as anode in the electrolysis of an aqueous solution containing ammonia and a lake-forming dye, whereby the article becomes coated with oxide of aluminum combined with the dye.

8. The process of providing an aluminum article with a colored coating, which comprises employing the article as anode in the electrolysis of an aqueous solution containing ammonia, ammonium sulphide and a lake-forming dye, whereby the articles become coated with a dense, absorbent and adherent coating of aluminum oxide combined with the dye.

9. An aluminum article having on its surface a dense adherent coating of oxide of aluminum which is resistant to abrasion and to injury by bending the article.

10. An aluminum article having on its surface a dense, absorbent and adherent coating of aluminum oxide combined. throughout its depth with a lake-forming dye, the coating being resistant to abrasion and to injury by bending the article.

11. An aluminum article having on its surface a dense adherent coating of electrodeposited oxide of aluminum which is resistant to abrasion and to injury by bondin the article.

In testimony whereof, I sign my name.

FULTON B. FLICK.

Witnesses:

JAMEs D. EDWARDS, ROBERT I. WRAY.

DISCLAIMER 1,526,127 .Fulton B. Flick, New Kensington, Pa. COATING ALUMINUM ARTICLES. Patent dated February 10, 1925. Disclalmer filed May 29, 1933, by the assignee,

Aluminum Colors Incorporated. Therefore, does hereby disclaim:

1. From the scope of claim numbered 5 of the said Letters Patent of the United States No. 1,526,127, all processes of coloring the surface of aluminum in which the coating of oxide of aluminum is not electrolytically produced.

2. From the scope of claim numbered 10 of the said Letters Patent of the United States No. 1,526,127, all aluminum articles in which the coating of aluminum oxide is not an electrolytic coating.

[Oflicial Gazette June 27, 1983.] 

